Wednesday, February 20, 2013

We're All Mammals part III

Mammals

Mammals (class
Mammalia) are a clade of warm-blooded amniotes. Among the features that
distinguish them from the other amniotes, the reptiles and the birds, are hair,
three middle ear bones, mammary glands in females, and a neocortex (a region of
the brain). The mammalian brain regulates body temperature and the circulatory
system, including the four-chambered heart. The mammals include the largest
animals on the planet, the rorqual whales, as well as the most intelligent such
as elephants as well as some primates and cetaceans. The basic body type is a
four-legged land-borne animal, but some mammals are adapted for life at sea, in
the air, in the trees, or on two legs. The largest group of mammals, the
placentals, have a placenta which feeds the offspring during pregnancy. Mammals
range in size from the 30–40 millimeter (1- to 1.5-inch) bumblebee bat to the
33-meter (108-foot) blue whale.

The word
"mammal" is modern, from the scientific name Mammalia coined by Carl
Linnaeus in 1758, derived from the Latin mamma ("teat, pap"). All
female mammals nurse their young with milk, which is secreted from special
glands, the mammary glands. According to Mammal Species of the World, 5,702
species were known in 2005. These were grouped in 1,229 genera, 153 families
and 29 orders. In 2008 the IUCN completed a five-year, 17,000-scientist Global
Mammal Assessment for its IUCN Red List, which counted 5,488 accepted species
at the end of that period. In some classifications, the mammals are divided
into two subclasses (not counting fossils): the Prototheria (order of
Monotremata) and the Theria, the latter composed of the infraclasses Metatheria
and Eutheria. The marsupials comprise the crown group of the Metatheria and
therefore include all living metatherians as well as many extinct ones; the
placentals likewise constitute the crown group of the Eutheria.

Homosexual
behavior in animals

Homosexual
behavior in animals refers to the documented evidence of homosexual and
bisexual behavior in various (non-human) species. Such behaviors include sex,
courtship, affection, pair bonding, and parenting among same-sex animal
pairings. A 1999 review by researcher Bruce Bagemihl shows that homosexual
behavior has been observed in close to 1,500 species, ranging from primates to
gut worms, and is well documented for 500 of them. Animal sexual behaviour
takes many different forms, even within the same species. The motivations for
and implications of these behaviors have yet to be fully understood, since most
species have yet to be fully studied. According to Bagemihl, "the animal
kingdom [does] it with much greater sexual diversity – including homosexual,
bisexual and nonreproductive sex – than the scientific community and society at
large have previously been willing to accept." Current research indicates
that various forms of same-sex sexual behavior are found throughout the animal
kingdom. A new review made in 2009 of existing research showed that same-sex
behavior is a nearly universal phenomenon in the animal kingdom, common across
species. Homosexual behavior is best known from social species. According to
geneticist Simon Levay in 1996, "Although homosexual behavior is very
common in the animal world, it seems to be very uncommon that individual
animals have a long-lasting predisposition to engage in such behavior to the
exclusion of heterosexual activities. Thus, a homosexual orientation, if one
can speak of such thing in animals, seems to be a rarity. One species in which
exclusive homosexual orientation occurs, however, is that of domesticated sheep
(Ovis aries). "About 10% of rams (males) refuse to mate with ewes
(females) but do readily mate with other rams."